Both of them have the same rights. If Martin had killed Zimmerman then he would also go free if he claimed Zimmerman attacked him and nobody could prove he didn’t. If Martin planned to stand his ground against the neighborhood watch, maybe he should have been armed too.
To say that they both have the same right as a defender is to place them both also as the aggressor. I don’t recall where a person can simultaneously be considered a perp and a victim. On any charge, it’s either/or.
Just because Zimmerman found himself outmatched doesn’t suddenly make him the victim.